Three lines of work will be carried out during the next year of the project. 1) The further development of the horseradish peroxidase histochemical (HRP) procedure for tracing efferent projections. Two specific goals are to be attempted: first, to perfect and rigourously test an immunohistochemical (as opposed to the current enzymatic) method for detecting HRP and second, to use HRP in the electron microscope to identify and characterize terminal populations arising from specific brain regions. This will be done using an existing method; the immunological procedure will also be tested. 2) Further work on the iontophoresis of HRP from recording microelectrodes. Beveled electrodes and purified enzymes will be used in an effort to develop a standard, reliable procedure; labelling will be carried out using intra as well as extracellular ejections. 3) In vitro studies on dendritic and azonal transport of HRP and other potential neural markers will be continued. Beyond continuing to develop the technology of maintaining slices of adult brain alive for extended periods in vitro we shall test the effects of local application of compounds known to block axoplasmic transport in the periphery on the bidirectional dendritic and axonal movement of HRP in the hippocampus. In summary, this work should provide 1) a sensitive method for tracing antrograde anatomical pathways in the light and electron microscopes, 2) a "final" version of the procedure for labelling cells with recording microelectrodes and 3) an in vitro procedure which should have broad applicability and which will be used to study transport effects in the mammalian brain.